1. Field of the Invention
The present invention relates to a light guide plate and a backlight unit, which are used in a transmissive or semi-transmissive liquid crystal display apparatus, an advertisement plate, an emergency guide light and the like.
2. Description of the Related Art
In recent years, color liquid crystal display (LCD) apparatuses are widely used in various devices such as a portable telephone, a portable personal computer, a portable liquid crystal television or a video integrated liquid crystal television and the like. The LCD apparatus is basically provided with a backlight unit and a liquid crystal element. Configuration of the backlight unit can be roughly classified into a direct type and an edge light type. For the direct type backlight unit, a light source is placed just beneath a liquid crystal element. For the edge light type backlight unit, a light source is placed on a side of a translucent light guide plate so that a light is emitted from the entire surface of the light guide plate. From the viewpoint of downsizing the LCD apparatus, the edge light type backlight unit is commonly used.
In such LCD apparatus, it is required to extend the operating time of the battery. However, the backlight unit used in the LCD apparatus consumes a high proportion of the electric power, which is an obstacle for extending the operating life of the battery. In order to extend the operating life of the battery and to improve the practical value of the LCD apparatus, it is very important to reduce the electric power consumption of the backlight unit as much as possible. However, if luminance of the backlight unit is decreased by reducing the electric power consumption of the backlight unit, display quality of the LCD apparatus may undesirably deteriorate. Therefore, in order to reduce the electric power consumption without decreasing the luminance of the backlight unit, development of a backlight unit having high luminance efficiency and evenness is advancing.
Currently, the most popular backlight unit includes a light source 1, a light guide plate 2, a diffusion film 3, upward prism sheets 4, 5 and a reflection sheet 6, as shown in FIG. 1. The light emitted from the light source 1, such as a light emitting diode (LED) and the like, enters in the light guide plate 2 from an entrance surface of the light, which is an end surface 2a of the light guide plate 2, and is guided inside the light guide plate 2. The light is reflected by a reflection element 122 having a plurality of grooves, a plurality of dots and the like. The reflection element 122 is formed on a reflection surface. The reflection surface is a bottom surface 2b of the light guide plate 2. The light is emitted in an oblique direction from an emitting surface defined by a top surface 2c of the light guide plate 2. The arrangement of the reflection element is designed such that the in-plane distribution of the luminance is even. For example, a surface density of the reflection element is low in the vicinity of the light source 1, and the surface density becomes higher as the distance from the light source 1 becomes longer. Consequently, the LCD apparatus has an even luminance.
However, the light is emitted in the oblique direction from the light guide plate 2. Thus, in order to effectively use the light, it is necessary to deflect and collect the light in a normal direction of the backlight unit. Therefore, the diffusion sheet 3 is placed on the light guide plate 2 so as to improve the evenness and to deflect the output light from the light guide plate 2 in the normal direction of the backlight unit. Moreover, as the lens sheet for controlling the direction of the lights and collecting the lights, two laminated upward prism sheets 4, 5 are provided. The prism sheets have a plurality of prism columns, each of which has a triangular cross section with an apex angle of about 90°, and are disposed on the diffusion sheet 3. The upward prism sheets 4, 5 are laminated such that each array direction of the prism columns is orthogonal to each other, so as to improve luminance efficiency of the backlight unit.
In the orthogonal configuration in which each array direction of the prism columns of the upward prism sheets 4, 5 is orthogonal to each other, directional control is performed for deflecting the emitted light from the light guide plate 2 to the normal direction of the backlight unit mainly by refraction on slant surfaces of the prism columns. Therefore, since apart of the light is laterally reflected and refracted, it is difficult to improve luminance efficiency. On the other hand, another part of the light is totally reflected in a downward direction out of the light guide plate 2. This light may be reflected by the reflection sheet 6 placed on a backside of the backlight unit and can be reused. The reused light may be emitted from a different position from the reflected position of the light guide plate 2. Thus, it is effective for resolving in-plane uneven luminance and increasing uniformity of the luminance. Since the arrangement shown in FIG. 1 provides a good balance between the efficiency and evenness of the luminance, this arrangement is widely used.
However, the orthogonal configuration of the upward prism sheet 4, 5 has limitations for improving the luminance efficiency, as discussed above. Thus, a light guide plate and a backlight unit, which are intended to provide high luminous intensity, have been developed.
A backlight unit having a downward prism sheet used as a lens sheet has been proposed (refer to JP No. 2739730). The proposed backlight unit is designed such that the diffusion film 3 and two upward prisms 4, 5 shown in FIG. 1 are replaced with a downward prism sheet 21, as shown in FIG. 2. The downward prism sheet 21 has a plurality of prism columns, each of which has a triangular cross section. The prism sheet 21 is downwardly disposed so that the prism columns face the top surface 2c of the light guide plate 2. The array direction of the prism columns is parallel to the end surface 2a of the light guide plate 2. The obliquely directed light emitted from the light guide plate 2 is refracted on a slant surface of each prism columns and totally reflected at another slant surface in the normal direction of the backlight unit. Thus, the emitted light from the light guide plate 2 is deflected in the normal direction of the backlight unit. In the configuration using the downward prism sheet 21, the directional light emitted from the light guide plate 2 is directly and totally deflected in the normal direction of the backlight unit. Thus, the front luminance efficiency may be increased in principle.
Furthermore, the number of parts in the lens sheet can be reduced to only one downward prism sheet 21. However, since the emitted light has a high directivity, it is difficult to reduce unevenness of the entering light and to assure even luminance. In practice, the diffusion film is stacked on the downward prism sheet 21 in many cases.
Recently, in the display of portable devices, LEDs are usually used as the light source. As shown in FIG. 3, in case of using LEDs as the light source 1, when the backlight is turned on and viewed from the front, an uneven light entrance region 33, in which a dark portion 31 and a bright portion 32 are clearly split due to the directional characteristics of the light emitted from the LEDs, occurs in the vicinity of the light entrance portion of the backlight. When reducing thickness and downsizing of the devices, the area ratio of a display area 34 in the backlight unit tends to be increased, and a distance LL of a light entrance area 35 between the entrance surface 2a and the end of the display area 34 is decreased. For this reason, minimization of the unevenness of the entering light is also an important subject.
The backlight unit of the downward prism sheet configuration disclosed in JP No. 2739730 has a disadvantage in that, since the output light from the light guide plate is emitted in the normal direction directly with one deflection without any redirect by reflection, a region where the uneven light is visible in the vicinity of the light entrance portion may increase. Also, even if the diffusion sheet is stacked on the downward prism sheet to improve the light unevenness, the light unevenness cannot be effectively reduced. Thus, in the actual situation, the configuration using a downward prism sheet is limited to the backlight unit in which the non-display area is large.
A backlight unit having higher luminance than the backlight unit of the downward prism sheet configuration has been proposed (refer to JP-A 2006-58844 (KOKAI)). The proposed backlight unit has a similar configuration shown in FIG. 2 and differs from the configuration shown in FIG. 2 in that a lens sheet having a saw-tooth shaped diffraction grating with a pitch of about 10 μm or less is used instead of the downward prism sheet. By designing the shape of the diffraction grating in detail, variation of the output angle of the light can be decreased with respect to variation of the incident angle of the light, and a high light collection efficiency is achieved. Also, by designing a shape of a diffraction grating so that an angle at which the diffraction efficiency of each wavelength of the three primary colors of blue, green and red is maximal corresponds to the normal direction of the emitting surface of the diffraction grating, the dispersion characteristic of the diffraction grating, may be suppressed. Moreover, the output light of the lens sheet is controlled by multiple interference of the diffraction lights transmitted through the plurality of concave and convex regions in the sawtooth shaped diffraction grating. Thus, the diffraction grating method has merit, even if one of the concave and convex regions is chipped, or if a foreign particle is on the diffraction grating, influence on the output light is small.
However, the region of uneven visible light may also increase in the diffraction grating method, because the output light from the light guide plate is directly emitted in the normal direction, although there is a difference between the diffraction and the total reflection, as compared with the downward prism method.
As a method of improving the problem of uneven light, as shown in FIG. 4, a technique to form a reflection element on the top surface 2c corresponding to the display area 34, and to form a dot pattern or a rough surface by sandblasting a surface 2d corresponding to the light entrance area 35 near the light entrance portion of the light guide plate 2 has been proposed (refer to JP-A 2006-286489 (KOKAI), and JP-A 2007-122958 (KOKAI)). In the proposed methods, the degree of uneven light is reduced by spreading the guided light inside the light guide plate 2 under the surface 2d near the light entrance portion by dispersion and dispersing the light emitted from the surface 2d near the light entrance portion.
However, since the above-discussed methods disperse the light, it is difficult to control the emitting direction of the light, and there is a disadvantage in that the light is dispersed in a direction that does not contribute to improve the problem of uneven light. That is, although the uneven light viewed from the front of the backlight unit is improved, the uneven light viewing from a specific direction other than the front direction is larger. Also, since the light is not efficiently used in the vicinity of the light entrance portion, luminance in the display area is decreased as a result. Thus, it is difficult to design a dot shape to achieve a balance between the light unevenness and luminance of the backlight unit, and to determine an appropriate sandblast condition.